The present invention is directed to a process for preparing lithium salts, and specifically, to an inexpensive process for preparing lithium salts from low cost lithium chloride and lithium sulfate.
Lithium salts have found utility in various applications. For example, lithium nitrate is known for use in ceramics, pyrotechnics, salt baths, heat exchange media, refrigeration systems, rocket propellants and specialized concrete applications. Another useful lithium salt, lithium bromide, is known for use in pharmaceuticals, air conditioning systems, low temperature heat exchange media, drying agent, refrigeration systems, batteries, medicine and as a humectant.
The conventional method of producing many lithium salts is to combine either lithium carbonate or lithium hydroxide with acids containing the desired anion for the lithium salt. For example, the conventional method of producing lithium nitrate is to react lithium carbonate and/or lithium hydroxide with nitric acid. Nevertheless, this process requires high purity raw materials and very expensive plant equipment and metal of construction for the handling of the highly alkaline lithium salts and concentrated nitric acid.
Similarly, the conventional method of producing lithium bromide is to react hydrobromic acid with lithium hydroxide or lithium carbonate. The saturated solution on cooling precipitates lithium bromide monohydrate, which can be dried to the anhydrous salt. Nevertheless, hydrobromic acid is a very irritating colorless gas that fumes strongly in moist air. Furthermore, hydrobromic acid is classified under DOT regulations, as highly corrosive, and highly irritative to eyes, skin and respiratory passages. Therefore, the production of lithium bromide using hydrobromic acid can be quite dangerous.
As described above, the use of highly acidic materials in the formation of lithium salts is undesirable as the materials are generally hazardous and require special equipment. Furthermore, the above processes generally do not produce high yields of the lithium salts being produced and therefore cannot be used where high purity lithium salts are desired. Therefore, there is a need to provide an inexpensive method of increasing the purity of the lithium salts without using highly acidic materials.
The present invention provides an inexpensive process for the preparation of lithium salts using lithium chloride and lithium sulfate. The process of the invention eliminates the use of highly acidic materials and thus reduces the cost of raw materials and the need for specialized equipment. The process produces lithium salts of desired purity without compromising safety in the production of these salts.
The process of the invention comprises preparing a lithium salt of formula LiX of desired or required purity by reacting a lithium salt selected from lithium chloride, lithium sulfate, and combinations thereof with NaX or KX in an aqueous, semiaqueous and/or organic solution and removing the precipitated solids from the semiaqueous solution to obtain a LiX solution of desired purity. Any combinations of solvents can be used in the invention but preferably the lithium salt and NaX or KX salt are reacted in a semiaqueous solution containing water and an organic solvent. Nevertheless, the semiaqueous reacting step can be preceded by an aqueous reacting step or can be replaced by successive aqueous and organic reacting steps. Typically, the organic solvent in the semiaqueous and organic solutions is selected from aliphatic ketones, aliphatic alcohols, and mixtures thereof. The process of the invention has been found especially useful for producing lithium salts of formula LiX wherein the solubility of LiX in the solution used is greater than the solubility of the sodium or potassium salts produced as by-products in the reacting step.
Use of sulfate salts as part of the starting materials or feed can require a cooling step to precipitate the resultant sodium and/or potassium sulfate salts, such as Na2SO4.10H2O, K2SO4, KLiSO4.H2O, NaLiSO4.H2O, and the like. The use of chloride salts as starting materials or feed can require higher temperatures to reject the resultant sodium salt NaCl and lower temperatures for the resultant potassium salt KCl from aqueous metathesis salting out. To purify LiNO3 and/or LiBr salt solutions, a solvent step metathesis step can be required for precipitation removal of sodium and/or potassium as chloride and/or sulfate salts.
In a specific embodiment of the invention, the process for preparing a lithium salt of formula LiX having a desired or required purity comprises dissolving lithium chloride, lithium sulfate, or a mixture thereof and a NaX salt, KX salt, or mixture thereof, in an aqueous solution and filtering the solution to remove the precipitated sodium and potassium salts. An aliphatic ketone, an aliphatic alcohol, or a mixture thereof, is then added to the solution to form a semiaqueous solution, and the precipitated sodium and potassium salts are again filtered from the solution. In addition, a monovalent cation sulfate salt can be added to the semiaqueous solution to cause the salting out or precipitation of undesired sodium and potassium salts from the solution thereby purifying the solution. The organic solvent is then removed and the resulting LiX solution of desired purity is recovered. The LiX solution can then be dried where solid LiX salt is desired as the end product or stored and sold in solution.
These and other features and advantages of the present invention will become more readily apparent to those skilled in the art upon consideration of the following detailed description which describes both the preferred and alternative embodiments of the present invention.
The process of the invention comprises preparing a lithium salt of formula LiX having a desired or required purity by reacting a lithium salt selected from lithium chloride, lithium sulfate, and combinations thereof with NaX or KX in an aqueous, semiaqueous or organic solution and removing the precipitated solids from the solution to obtain a LiX solution of desired purity. Any combination of solvents can be used in accordance with the invention. Preferably, a semiaqueous solution is used containing water and an organic solvent. The reaction can also be initiated in an aqueous solution and then continued in either a semiaqueous or an organic solution to provide the LiX salts of the invention. As used herein, and as will be appreciated by the skilled artisan, the term xe2x80x9csaltxe2x80x9d unless otherwise specifically defined can refer to a salt provided in solid or liquid form (for example as brine solutions).
The lithium chloride and lithium sulfate salts used in the process are inexpensive and readily available in anhydrous form, hydrated form, or in solution. Typically, the lithium chloride is obtained in purified form as a liquid as well as an anhydrous salt from geothermal brine deposits such as those in Chile and Argentina by selective adsorption and solar evaporation. Lithium sulfate is obtained from spodumene ore by conventional roasting, acid leach and purification steps, and from brine by conventional routes. The NaX or KX salts are also available in anhydrous form, hydrated form, or in solution and can be available as mixtures of NaX and KX. Advantageously, the lithium chloride, lithium sulfate, NaX and KX can be used in liquid and well as solid form. Thus, where the transport of the raw materials is not a concern, the lithium chloride, lithium sulfate, NaX and KX can be used in liquid form and thus do not require drying. As a result, the cost of the raw materials used in the process is reduced.
The process of the invention is preferably initiated by dissolving lithium chloride, lithium sulfate, or a mixture thereof and a NaX salt, KX salt, or a mixture thereof, in an aqueous solution. The dissolution of these reactants occurs by dissolving lithium chloride or lithium sulfate in a NaX or KX solution, dissolving NaX or KX in a lithium chloride or lithium sulfate solution, or mixing an aqueous solution of lithium chloride or lithium sulfate with an aqueous solution of NaX or KX. In addition, the solution can be heated and/or agitated to facilitate the dissolution of the salts in the solution. Preferably, the dissolving step is conducted at between 0 and 120xc2x0 C. and under atmospheric or subatmospheric pressure (5 to 25 mm Hg).
As the salts are combined in solution, the lithium chloride and lithium sulfate reacts with the NaX and KX salts in a substitution or metathesis reaction to form LiX as the desired end product and NaCl, Na2SO4, KCl, K2SO4 and the like as by-products. The LiX is preferably more soluble in the aqueous solution than the sodium and potassium salts, i.e. the salt by-products and any excess NaX or KX present in the solution, thereby causing the sodium and potassium salts to precipitate out of solution before the LiX salt. The solution is often concentrated such as by evaporating off the water or other means to thereby increase precipitation of the sodium and potassium salts. Preferably, the solution is concentrated such that the solution contains from greater than 0 to about 4 moles of water per mole of LiX. More preferably, the aqueous solution contains between about 1 mole and 3 moles of water per mole of LiX. The amount of water in solution is preferably sufficient to keep the LiX in solution below or close to saturation at the operating temperature. Typically, the pH of the aqueous solution is between about 5 and 10, preferably between about 6 and 8. In addition, where the solution has been heated, the solution can be cooled to help precipitate more sodium and potassium salts from the solution. Once the sodium and potassium salts precipitate, these salts can be removed from the aqueous solution by continuously filtering the solution or other suitable means.
Once the partially precipitated sodium and potassium salts have been removed from the aqueous solution for further purification, an organic solvent is added to the aqueous solution to form a semiaqueous solution. Alternatively, the water can be removed completely in the solution and the organic solvent added to form an organic solution. Typically, the solution is agitated and can be heated to facilitate the formation of the semiaqueous or organic solution. It can also be beneficial in the case of the semiaqueous solution to dilute the solution by adding water to the solution to bring the amount of dissolved solids in the solution to less than 50 weight percent prior to adding the organic solvent. The preferred organic solvents for use in the process of the invention are aliphatic ketones (e.g. primary and secondary C1-C10 ketones), aliphatic alcohols (e.g. primary and secondary C1-C10 alcohols), and mixtures thereof. Exemplary ketones and alcohols include without limitation acetone, dimethylketone, diethylketone, methanol, ethanol, propanol, pentanol, hexanol, and the like, and preferably is acetone, methanol and/or ethanol, and more preferably acetone and/or methanol.
Preferably, the amount of organic solvent in the semiaqueous or organic solution is greater than 0 moles and less than 10 moles per mole of LiX. More preferably, the amount of organic solvent is between about 0.25 moles and about 3 moles per mole of LiX. The amount of water present in the semiaqueous solution is as described above. Preferably, the amount of water (semiaqueous solution) and organic solvent in the semiaqueous or organic solution is sufficient to keep the LiX in solution below or close to saturation at the operating temperature. The addition of the organic solvent and filtration of the precipitated solids is typically conducted at between 0 and 60xc2x0 C., preferably, between about 0xc2x0 C. and 30xc2x0 C., and under atmospheric or subatmospheric pressure (0 to 25 mm Hg). In addition, the pH of the semiaqueous solution is typically between about 5 and 10, preferably between about 7 and 9.
Once the organic solvent has been added to the solution, additional sodium and potassium salts precipitate out of the semiaqueous solution. In addition to the organic solvent, a sulfate salt having a monovalent cation, e.g., an alkali metal sulfate, such as but not limited to Na2SO4, Na2SO4.10H2O, LiSO4, LiSO4.H2O, KLiSO4.1/2H2O, K2SO4, and NaLiSO4.H2O, and the like, can be added to the solution with the organic solvent or prior to drying the aqueous solution to facilitate precipitation of the sodium and potassium salts. Preferably, the added sulfate salt is sodium sulfate or potassium sulfate to correspond to the salt originally added in the process to provide the X anion for the LiX salt. The precipitated salts are removed from the semiaqueous solution such as by filtering. In addition, where elevated temperatures are used for the semiaqueous solution, the solution temperature can be decreased to cause greater precipitation of the sodium and potassium salts.
Once the precipitated solids have been substantially removed from the lithium solution, the organic solvent is removed next from the solution. The organic solvent can be recycled into the process. The recovery of the organic solvent from the solution is preferably facilitated by concentrating the solution such as by heating the solution to evaporate (distillation) the organic solvent. The resulting LiX solution or salt can have a purity of 95% or greater, preferably 99% or greater, and is recovered in the process. When a semiaqueous solution is used, the LiX solution can then be concentrated to increase the amount of solids in the solution or dried to form a LiX salt of desired purity.
Although the process of the invention is described as a two-step process in which precipitation occurs first in an aqueous solution and then in a semiaqueous or organic solution, the process can be conducted as a one-step process wherein only the semiaqueous solution is used under the semiaqueous conditions described above. In particular, the one-step process is preferred wherein LiX is less soluble in water than the sodium and potassium salts but more soluble in the semiaqueous solution than the sodium and potassium salts. In addition, the process of the invention can be conducted in a batch, semi-batch or continuous process to produce the LiX salts. Preferably, for the large-scale production of lithium salts, a continuous or semi-batch process is used in accordance with the invention.
The present invention can be used to form various LiX salts. Preferably, as will be readily understood to one skilled in the art, the process of the invention is used to produce LiX salts having a greater solubility in the solutions used than the sodium or potassium salts present in these solutions. In particular, the process of the invention can be used to produce a lithium salt of formula LiX wherein X is an anion selected from nitrate, bromide, chloride, iodide, borate (tetraborate), acetate, pentaborate, acetylsalicylate, amide, benzoate, chlorate, perchlorate, chloroplatinate, chromate, citrate, dichromate, fluosilicate, fluosulfonate, formate, hydroxide, hypochlorite, iodate, lactate, permanganate, methoxide, molybdate, nitrite, niobate, oxalate, oxide, salicylate, sulfate, sulfide, sulfite, tartrate, thiocyanate, dithionate, tungstate, and vanadate. Preferably, the process of the invention is used to make lithium nitrate or lithium bromide.
The process of the invention allows LiX to be produced from NaX and KX salts which are generally more readily available and are therefore less expensive than the related LiX salts. For example, sodium nitrate (also called Chilean nitrate or Chile saltpeter) is abundantly available in South American salt deserts and potassium nitrate is made from sodium nitrate in Chile. Sodium bromide is recovered as a by-product from solutions obtained during bromination, bromo-oxidation and hydrolysis of organic bromide compounds. Furthermore, large amounts of potassium bromide are made from the neutralization of HBr with KOH or K2CO3. Any excess NaX and KX salts which are recovered in the process can be separated and recycled into the process.
Another advantage of the process of the invention is that high purity raw materials are not required to produce the LiX solutions or salts having the desired or required purity. Specifically, because undesired salts generally precipitate out of the solution during the process of the invention, impurities can be present in the raw materials. Thus, more expensive high purity raw materials are not necessary in order to produce LiX having a desired purity according to the invention.
A further advantage of the process is that the salt by-products, i.e., the sodium and potassium salts, are useful chemicals and thus are not discarded as waste. For example, potassium chloride is a raw material for manufacturing potassium nitrate from Chile saltpeter (NaNO3) and potassium sulfate is a very valuable potash fertilizer. In addition, NaCl (table salt) and Na2SO4 (salt cake) have uses which are well known in the art.
The present invention provides an inexpensive process for the preparation of lithium salts using lithium chloride and lithium sulfate. The process of the invention eliminates the use of highly acidic materials and thus reduces the cost of raw materials and the need for specialized equipment. Therefore, the process produces lithium salts having a desired or required level of purity without compromising safety in the production of these salts. Additionally, lithium chloride and lithium sulfate can be used in both solid and solution form in the process and thus do not have to be dried out prior to use. Advantageously, the by-products of the process of the invention have other useful applications and thus the process produces little waste.
The present invention will now be further illustrated by the following non-limiting examples. All percentages unless otherwise indicated are on a per weight basis.